Field of the Invention
The present application relates to a double-stranded nucleic acid having an activity of suppressing the expression of a target gene by means of an antisense effect, and more particularly, to a double-stranded nucleic acid including an antisense nucleic acid that is complementary to the transcription product of a target gene and contains a region comprising four or more contiguous bases, and a nucleic acid that is complementary to the foregoing nucleic acid.
Related Background Art
In recent years, oligonucleotides have been a subject of interest in the on-going development of pharmaceutical products called nucleic acid drugs, and particularly, from the viewpoints of high selectivity of target gene and low toxicity, the development of nucleic acid drugs utilizing an antisense method is actively underway. The antisense method is a method of selectively inhibiting the expression of a protein that is encoded by a target gene, by introducing into a cell an oligonucleotide (antisense oligonucleotide (ASO)) which is complementary to a partial sequence of the mRNA (sense strand) of a target gene.
As illustrated in FIG. 1 (upper portion), when an oligonucleotide comprising an RNA is introduced into a cell as an ASO, the ASO binds to a transcription product (mRNA) of the target gene, and a partial double strand is formed. It is known that this double strand plays a role as a cover to prevent translation by a ribosome, and thus the expression of the protein encoded by the target gene is inhibited.
On the other hand, when an oligonucleotide comprising a DNA is introduced into a cell as an ASO, a partial DNA-RNA hetero-duplex is formed. Since this structure is recognized by RNase H, and the mRNA of the target gene is thereby decomposed, the expression of the protein encoded by the target gene is inhibited. (FIG. 1, lower portion). Furthermore, it has been also found that in many cases, the gene expression suppressing effect is higher in the case of using a DNA as an ASO (RNase H-dependent route), as compared with the case of using an RNA.
On the occasion of utilizing an oligonucleotide as a nucleic acid drug, various nucleic acid analogs such as Locked Nucleic Acid (LNA) (registered trademark), other bridged nucleic acids, and the like have been developed in consideration of an enhancement of the binding affinity to a target RNA, stability in vivo, and the like.
As illustrated in FIG. 2, since the sugar moiety of a natural nucleic acid (RNA or DNA) has a five-membered ring with four carbon atoms and one oxygen atom, the sugar moiety has two kinds of conformations, an N-form and an S-form. It is known that these conformations swing from one to the other, and thereby, the helical structure of the nucleic acid also adopts different forms, an A-form and a B-form. Since the mRNA that serves as the target of the aforementioned ASO adopts a helical structure in the A-form, with the sugar moiety being mainly in the N-form, it is important for the sugar moiety of the ASO to adopt the N-form from the viewpoint of increasing the affinity to RNA. A product that has been developed under this concept is a modified nucleic acid such as a LNA (2′-O,4′-C-methylene-bridged nucleic acid (2′,4′-BNA)). For example, in the LNA, as the oxygen at the 2′-position and the carbon at the 4′-position are bridged by a methylene group, the conformation is fixed to the N-form, and there is no more fluctuation between the conformations. Therefore, an oligonucleotide synthesized by incorporating several units of LNA has very high affinity to RNA and very high sequence specificity, and also exhibits excellent heat resistance and nuclease resistance, as compared with oligonucleotides synthesized with conventional natural nucleic acids (see JP 10-304889 A). Since other artificial nucleic acids also have such characteristics, much attention has been paid to artificial nucleic acids in connection with the utilization of an antisense method and the like (see JP 10-304889 A, WO 2005/021570, JP 10-195098 A, JP 2002-521310W, WO2007/143315, WO2008/043753 and WO2008/029619).
Furthermore, when n oligonucleotide is applied to a drug, it is important that the relevant oligonucleotide can be delivered to the target site with high specificity and high efficiency. In addition, as methods for delivering an oligonucleotide, a method of utilizing lipids such as cholesterol and vitamin E (Kazutaka Nishina et al., Molecular Therapy, Vol. 16, 734-740 (2008) and Jurgen Soutscheck et al., Nature, Vol. 432, 173-178 (2004)), a method of utilizing a receptor-specific peptide such as RVG-9R (Kazutaka Nishina et al., Molecular Therapy, Vol. 16, 734-740 (2008)), and a method of utilizing an antibody specific to the target site (Dan Peer et al., Science, Vol. 319, 627-630 (2008)) have been developed.